An internal combustion engine in a vehicle may operate over a wide range of speed and load. In a boosted gasoline engine operating at a stoichiometric air-fuel ratio, high engine load may result in undesirably high exhaust temperatures. Such temperatures may accelerate materials ageing in the engine system—especially in the turbine and in exhaust-aftertreatment components.
One strategy to reduce the exhaust temperature of a gasoline engine during a high-load transient is to enrich the air-fuel mixture. Above the stoichiometric ratio, liquid fuel injected into an engine cylinder does not contribute to combustion, but to evaporative cooling of the combustion gasses therein. In addition, the excess fuel may be reformed endothermically in the cylinder, providing further cooling. This strategy may erode fuel economy, however, as the excess fuel provides no power.
To address this issue, the inventors herein disclose a series of solutions in which exhaust-gas recirculation is applied aggressively during high-load conditions, while the air-fuel mixture is enriched. One specific embodiment provides a method for operating a boosted gasoline engine. The method includes diluting an intake air charge of the engine to a first level of dilution when operating at a stoichiometric air-to-fuel ratio. The method also includes, in response to a condition of excessive exhaust temperature downstream of the engine, diluting the intake air charge to a second, greater level of dilution while operating at an enriched air-to-fuel ratio.
Certain advantages of EGR in a gasoline engine at high load have been observed previously. U.S. Pat. No. 8,001,779, for example, provides a hybrid high- and low-pressure EGR system, which extends the usefulness of EGR over a wide range of speed and load. This reference recognizes that EGR applied at high load can reduce the need for enrichment to protect exhaust-system components. However, it does not recognize the synergy that may result from the combined use of enrichment and aggressive EGR during high-load transients.
By contrast, the present disclosure demonstrates that application of certain forms of EGR can be used, together with enrichment, as a remedy for the fuel-economy losses that result from enrichment. In this manner, the exhaust-system may be protected during high-load transients, but without undue loss of fuel economy.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address the problems or disadvantages noted herein.